Cancer of the colon and rectum continues to kill within 5 years more than 50% of the people in whom it occurs. We have embarked on a screening program for the detection of early colorectal cancer and colonic adenomas using Hemoccult slides in asymptomatic patients. To date more than 7,000 patients have been screened and a control group of 6,500 patients has been developed. Compliance has been high. The rate of positive slides has been approximately 1% and in most patients only one or two slides have been positive. Investigation in patients with positive slides has revealed the following: adenomas, 40%; diverticulosis only, 18%; hyperplastic polyps, 16%; cancer, 12.5%; normal, 7%; miscellaneous, 6.5%. Approximately 1/3 of the polyps were detected only by colonoscopy and not visualized by barium enema. Directed endoscopic biopsy and cytology has been accurate in the diagnosis of cancers that were observed but lavage cytology has not been useful. CEA concentrations in colonic lavage has generally correlated with colonic pathology. Isotope incorporation with tritiated thymidine in biopsies and lavage have indicated a high incidence of DNA-synthesizing cells on the surface of the mucosa in patients with a variety of colonic abnormalities. Cancers detected in asymptomatic Hemoccult-positive patients have demonstrated an early pathological stage. Future plans include: expansion of all populations to yield more significant data; rescreening of study populations for accurate determination of false-negative rate of Hemoccult slide-testing; tracking of patients for colonic disease; cost-effectiveness; compliance; survival of the patients with cancer detected by Hemoccult slide-testing as compared to the controls; assessment of metachronous rates of cancer and polyps; evaluation of enzymes in colonic lavage; further studies of CEA in lavage before and after surgery; further isotopic incorporation studies with thymidine especially after removal of polyps and cancer, and sequentially in a variety of colonic abnormalities.